The somatomedin (SM) hypothesis states that growth hormone (GH) acts via an intermediary substance, SM, to illicit its biological effects. Several growth promoting substances have been isolated. These include SMA, B and C and non-suppressible insulin like activity, soluble, and multiplication stimulating activity. In addition to their metabolic activities in vivo, several of these peptides are mitogenic in vitro. The chemical nature of SM-C, which stimulates sulfation and DNA synthesis in rat cartilage, is not known to any great extent. No biological studies have been done with pure preparations of SM-C. We propose to determine the chemical nature of SM-C, i.e., amino acid composition and sequence, molecular weight and N- and C-terminal amino acid residues. Studies using pure SM-C will be performed to gain insight regarding its mitogenic actions in vitro in cultured fibroblasts and its biological actions in vivo in hypophysectomized rats. The binding of 125I-SM-C to the presumptive cell membrane receptor will be studied. The specificity and kinetics of binding will be examined in cultured fibroblasts. Antibodies to SM-C will be generated and employed as a radioimmunoassay and, after immobilization to Sepharose, as a means of purifying additional SM-C. A water-soluble SM-C/dextran complex will be prepared and the kinetics of its biological activities will be compared with native SM-C. Studies will be performed to demonstrate a specific carrier protein of 125I-SM-C in vivo.